This invention relates to an actuator for controlling the amount of fluid delivered through a valve arrangement, and more particularly, to an actuator for independently controlling the application of fluid (e.g. water, steam) in discrete zones across a papermaking machine.
Paper Production
In the modern production of paper, a continuous fiber/water slurry is formed as a moving web. As the slurry moves down the paper machine the water is removed to leave the fiber which forms the paper sheet. The first section of the paper machine drains the water under the influence of gravity (on the fourdrinier table) and produces a web with sufficient strength to be self-supporting to feed it into a press section. The second section of the paper machine presses the paper web and squeezes the water from the sheet. This section typically consists of a series of rolls forming press nips between them through which the paper web is fed. After pressing removes all the water that it can, the remaining moisture in the web must be evaporated. The third section of the paper machine evaporates the remaining moisture in the paper web down to the final level desired for the grade of paper being produced.
During the production of paper it is important that a consistent quality be produced and maintained. Of the many paper parameters, the moisture content is probably the most basic. It is not only important that the overall moisture level be controlled, but also that the moisture distribution throughout the sheet be controlled both in the moving (machine) direction (MD) and in the width (cross-machine) direction (CD). Variation in moisture content of the sheet will often affect paper quality as much or more than the absolute moisture content. There are numerous influences on the paper machine that can cause variation of the moisture content; in particular in the cross machine direction. Wet edges and characteristic moisture profiles are common occurrences on paper machines. Thus a number of actuator systems have been developed to offer control of the moisture profile during paper production.
Conventional actuator systems for controlling the moisture profile across the sheet in paper machines work by selectively delivering steam or spraying water onto the paper web during production. If steam is used, it is added before the press nips with the effect of increasing the temperature of all of the moisture in the web. The added temperature makes the water removal by pressing much more effective; the added moisture removal being much greater than the added moisture of steam condensation. If water is sprayed onto the web, it is done in the evaporating section. The added water to the surface can be used to even out the moisture variance across the web. It can have the added effect of locally cooling the web to prevent damaging overheating. Water sprays are generally used for quality improvements while steam showers are used for both production and quality improvements.
Steam Shower Systems
Profiling steam showers deliver a variable distribution of steam in zones across the paper web. Each zone needs an independent actuator to control the volume of flow in that area. Traditionally the actuator used for such a zone control has controlled the steam flow rate by positioning a steam valve in response to a pneumatic signal. The pneumatic signal is varied, typically from 6 psig to 30 psig, to set an amount of valve opening. The pneumatic control portion of the actuator is separated from the steam valve portion.
In this invention a pneumatic signal is varied to directly control the steam pressure at discrete zones across the paper web. An orifice determines the steam flow from its controlled pressure chamber to the paper web area. This approach allows for a smaller actuator and a full shutoff design.
Moisture Spray Systems
Moisture spray actuator systems used in paper machines are designed to apply a profile of moisture spray in the cross-machine direction to counter an undesirable moisture profile in the paper web. Thus these systems consist of a series of actuator modules capable of independently adjusting the amount of spray in discrete adjacent zones in the cross-machine direction. Control of the flow at each zone is made from a logic decision off the machine via a signal sent to that zone position. How this signal is handled becomes an important consideration for such actuator systems.
The moisture spray systems used in paper machines are designed around spray nozzle characteristics. The nozzle is the device that breaks the water particles into a fine droplet size. These nozzles typically use either the hydraulic pressure of the water or use a separate air pressure line to produce the droplets. Other techniques may include such technologies as ultrasonics to produce droplets.
Hydraulic designs use the water pressure directly to break up the water droplets into a spray mist. Typically this technique is limited in how small a particle size it can produce. A change in flow rate affects the mist characteristics (particle size, spray pattern, etc.) of hydraulic nozzles reducing its turndown capability making these nozzles most effective at a single flow rate. These designs need accurately machined tiny openings that become subject to impurities in the feed water. Any partial plugging or blocking of a nozzle opening affects the volume flow as well as the spray pattern and causes the nozzle to lose its mist effect.
Pneumatic designs use compressed air to break up the water droplets into a spray mist. The mist is carried by the compressed air flow to the web surface. The nozzle openings for the water are not as critical to the spray pattern giving them a greater turndown capability. However it is typical that the average particle size will vary (increase) as the water flow is increased. Any partial plugging of the water nozzle opening affects the volume flow but often not the spray pattern. Both the water and air must be provided without impurities to maintain proper operation.
It is of particular interest to make a system that interfaces readily to a programmable logic controller (PLC) or computer. The conventional method of control for each zone has been to use multiple solenoid-operated valves. Each valve can be optimized for spray particle size at a particular water flow rate. Also, solenoid valves give reasonable assurance of 100% shutoff. These multiple valve groupings open the volume flow in a binary manner such that the first solenoid valve allows the minimum flow, the second solenoid valve allows twice the minimum flow, the third solenoid valve allows four times the minimum flow and so on. Thus 4 solenoids are used in combinations to give 16 discrete flow settings while 5 solenoids give 32 discrete flow settings. Nozzles are sized to optimize the spray pattern and particle size for the particular flow.
Locating the zone control solenoid actuators local to the spray nozzles (out over the paper machine) gives the most compact overall design. A common water header (and a common air header when pneumatic designs are used) supplies all zones. Typically a block encompassing the multiple solenoid-operated valves is mounted in each zone. Wiring is fed out to the individual on-machine solenoids. This approach has the disadvantage of placing the electrical solenoids in a very harsh environment. Failures of solenoids are frequent in the very hot and humid environment and replacement of solenoids can require an expensive paper machine shutdown.
Zone control solenoid actuators have, in some systems, been located off the machine with the water piped to the individual zones for spraying. These systems put the electrical solenoids in a controlled environment and give them accessibility. Pressure drops of water (and air) from the control cabinet to the machine zones must be addressed. The space required on the machine is about the same due to the tradeoff of solenoids for individual piping. However, the space required off the machine is greatly increased to accommodate the extra piping, etc.
From the above discussion, it is clear that existing water spray systems have a number of limitations. The first difficulty is the amount of space required for the portion of the system located on the paper machine. It is typical to find that there is little available space between rolls, carrier felts, paper web, etc. A smaller space requirement for the moisture spray system would mean many more opportunities to optimize the location of equipment on the paper machine. Multiple nozzles per zone require greater space to fit the nozzles, associated piping and solenoids. This introduces more weight needing support, which in turn, requires a greater support structure and again more weight.
The binary control strategy of multiple solenoid valves per zone limits the resolution of the flow control. Although the resolution is generally considered xe2x80x9cgood enoughxe2x80x9d, the ability to optimize control is limited. In addition multiple solenoids bring multiple potential points of failure along with their additional cost. Clearly fewer components would give fewer opportunities for failure.
It is also recognized that the solenoid operator is a low lifetime component in this application. The paper machine is operated continuously 24 hours a day with only one or two planned shutdowns a year. Since it is extremely expensive to lose production, installed equipment is desired to have a 10 to 20 year lifetime with little or no servicing. Actuator reliability is critical.
Regulation of spray water for continuous settings has limitations when desired in a harsh environment. Generally the spray nozzles, as used in the papermaking application described above, require a very low flow rate. Remote regulation of the spray water supply would require flow control rather than pressure control to overcome the difficulty of low flow, long transport lines and various pressure drops. In addition, since the water is flowing, a continuous regulation would be required which can be costly. Regulation of the spray water pressure local to the spray nozzle would give accurate flow rate characteristics. However, in a harsh environment, a controller needs to be reliable and use a reference medium that is inherently rugged. Interestingly, the reference medium does not need to flow continuously but simply hold a reference setting. This realization gives some flexibility to the potential control strategies that can be employed. The present invention applies this concept to create an infinitely variable actuator.
To address the shortcomings of the prior art, we have developed a novel actuator for controlling the delivery of moisture to a surface. The actuator can be used in conjunction with a spray nozzle in a water spray system for a paper machine. The actuator also finds application in a steam control flow valve for use in a steam shower on a paper machine. This invention uses a single nozzle actuator assembly at each zone with full shutoff capability. More importantly, the actuator of the present invention incorporates a novel concept for a fully proportional actuator. Resolution of this new actuator is limited only by the control signal to the actuator and not by the actuator itself. The small size and weight of the actuator allows for a minimum space requirement.
Accordingly, the present invention provides an actuator for controlling the flow of fluid from a fluid source comprising:
a housing having a first inlet connectable to the fluid source and a second inlet connectable to a pressure source;
a piston movable within the housing;
a flexible seal extending between the piston and the housing to define a first region in sealed communication with the first inlet and a second region in sealed communication with the second inlet, the piston moving within the housing in response to the difference in pressures between first and second regions;
a valve member adapted to move with the piston to open and close the first inlet to vary the pressure in the first region; and
at least one orifice in sealed communication with the first region to permit the exit of the flow of fluid and provide resistance to the fluid flow so that the pressure in the first region builds to match proportionally the pressure at the second inlet, the pressure from the pressure source at the second inlet providing a signal to control the pressure in the first region feeding the at least one orifice to determine the flow passing the at least one orifice without regard to the exact position of the valve member.
Preferably, the flexible seal comprises a metal bellows structure.
In the first embodiment, the actuator of the present invention is used to control a standard off-the-shelf hydraulic water spray nozzle. Alternately, the actuator of the first embodiment can be used to control the steam pressure feeding an orifice to control the steam flow to a steam shower zone. The bellows structure of the actuator operates as a regulator to control the pressure of the fluid fed to the nozzle (or orifice) and, in this way, controls the water (or steam) flow to the zone. The reference pressure for the bellows structure is a pneumatic pressure signal generated off machine.
The actuator of the present invention is preferably used in a system which includes a common header extending across the paper machine to carry the fluid being delivered to the paper web, e.g. spray water or steam. Preferably, this header has regular attachment points where the actuator of the present invention and the outlet for each zone attaches. The outlet can be a water spray nozzle or a steam orifice. The header is fabricated and machined identically at each zone to accurately locate the spray nozzle or steam orifice, feed the spray water or steam to the actuator location and to fit the actuator in place. A separate small diameter tube is brought out to each actuator to deliver the pneumatic control signal.
The actuator regulates the pressure of the fluid (spray water or steam) using the pneumatic control signal for reference. The design is made to allow for a non-zero kickoff pressure. As such, the control air can operate with a different minimum pressure than the spray water or steam allowing a typical 6-30 psig range of pneumatic air to produce a 0-24 psig range of spray water or steam. A different range of pneumatic air control pressures may be used to match a particular operating range of steam or water pressures.
The bellows structure of the actuator separates the pneumatic control air and the fluid being delivered to the web and is positively sealed (welded or soldered) to prevent leakage. The stroke of the bellows structure is extremely small as it serves only to balance pressures. This means that the inherent spring rate of the bellows or a separate spring can be used for pre-loading the actuator (giving a non-zero xe2x80x9ckickoffxe2x80x9d) but will have a negligible effect on the operation. The result is a highly linear actuator response feeding the outlet of the actuator.
In a further embodiment, the present invention provides an actuator that is used for controlling the larger steam flows to a steam shower. This embodiment of the actuator uses a significantly larger inlet opening which is required to allow for these larger steam flow rates. A larger inlet opening will exert a greater back pressure on the bellows structure and negatively affect its accuracy and linearity. In the second embodiment, a double inlet opening is used and arranged such that only the difference in area of the two inlet openings affects the back pressure on the bellows structure. Thus, a significantly larger inlet opening and flow rate can be accommodated without losing accuracy and linearity.
In a still further aspect, the present invention provides an actuator for controlling the flow of fluid from a fluid source comprising:
a housing having a first inlet connectable to the fluid source, a second inlet connectable to a first pressure source, and a third inlet connectable to a second pressure source;
a piston movable within the housing;
a first flexible seal extending between the housing and the piston to define a first region in sealed communication with the first inlet;
a second flexible seal extending between the housing and the piston to define a second region in sealed communication with the second inlet;
a third flexible seal extending between the housing and the piston to define a third region in sealed communication with the third inlet;
the piston moving within the housing in response to pressure differences within the first, second and third regions to a position such that the forces exerted by the first region on the piston are balanced by the forces exerted by the second and third regions;
a valve member adapted to move with the piston to open and close the first inlet to vary the flow of fluid from the fluid source; and
an outlet from the housing in communication with the first inlet to permit the exit of the flow of fluid whereby varying the pressure from the pressure source at the second inlet provides a signal to move the position of the piston to control the pressure of fluid from the outlet.
Once again, the flexible seals preferably comprise metal bellows structures.
According to this further aspect, the present invention provides an actuator that is used to control two separate flows, one of water and the other of air, in a predictable ratio using a single pneumatic control signal. In this embodiment, the pneumatic control signal is again typically a 6-30 psig range although other signal ranges can be used. However, at different water flow rates, the pressure and flow rate of atomizing air adjusts to maintain a substantially constant spray particle size of the water droplets. By using multiple bellows of relative size and specific arrangement, a single control signal can be used to effect the combination of final water flow rate and air/water ratio to maintain the optimum water droplet size.
The present invention also provides an actuator for use with steam in relatively low flow rate applications. The valve opens only enough to pass sufficient volume flow to maintain the balance pressure with the pneumatic control signal. It is recognized that if the supply pressure of steam is sufficient, the flow through the valve reaches sonic speeds. Such high velocities can cause surface wear. To mitigate this effect the fourth embodiment includes one or more reduced area passages in the inlet flow path which cause pressure drops prior to the inlet valve itself. In this manner the pressure drop across the inlet valve is reduced which causes the valve to open more and reduces the flow velocity through the valve.